Process for corrosion control



United States Patent 3,406,042 PROCESS FOR CORROSION CONTROL Arthur R.Belyea, Norwalk, Conn., assignor to Consolidated Edison Company of NewYork, Inc., New York,

N.Y., a corporation of New York No Drawing. Filed Dec. 14, 1965, Ser.No. 513,821 9 Claims. (Cl. 117-134) ABSTRACT OF THE DISCLOSURE Processof inhibiting corrosion in steam generating boilers, and the like, usingquaternary ammonium salts.

This invention is concerned with a new and useful process for theinhibition of corrosion of metallic parts which are subject to contactby products of combustion of sulfur containing fuels. More particularly,it is con cerned with the inhibition of acid corrosion caused bydeposition of sulfuric acid arising from condensation of combustionproducts on the surfaces of metallic parts which are reactive withsulfuric acid.

Fuels commonly used in commercial combustion equipment, for exampleboilers for generating steam, may contain varying proportions of sulfur.Many solid fuels, for example contain from about 0.5 to 3% sulfur.Liquid petroleum fuels may contain from about 2 to about 4 /2 sulfur.When these fuels are burned in the presence of oxygen the principalproducts of combustion which appear in the flue gases are water, carbondioxide, sulfur dioxide and sulfur trioxide. Normally the combustionproducts are not a corrosion problem While the boiler is operating on ahigh load since their temperature is well above the condensationtemperature of even the least volatile component.

Sulfuric acid arises in the flue gases as a result of sulfur trioxidedissolving in water which condenses from the flue gases when theirtemperature is at or below the dew point, i.e. the acid-water saturationtemperature. This may occur during the period when the boiler isstarting up but before the temperature of the flue gases exceeds the dewpoint. It may also occur While the boiler is being banked. During thisperiod the temperature of the flue gases decreases and may reach the dewpoint, i.e. 350- 400 F.

As a matter of economy, many large boilers are equipped in the fluestack with economizers or with air heaters. The function of thesedevices is to abstract the heat content from the flue gases so as tosave combustion costs. An economizer functions by preheating the waterwhich is to be converted to steam in the boiler. The air heaterfunctions by heating the air which is to be used as the source of oxygenin the combustion of the fuel. These units are generally constructed ofiron or other sulfuric acid sensitive metals. The condensation ofsulfuric acid on their surface produces ferrous sulfate which because ofits poor heat transfer properties compared to iron materially decreasesthe efficiency of the unit.

In many commercial operations the boiler may be started up or bankedseveral times in the course of an extended period of operation. Forexample, if the boiler "ice is one which is utilized to service a largeurban area containing many manufacturing plants or offices as well asresidential sections, the total steam which must be produced in thecourse of a normal day will reach peak during working hours. The needfor steam is materially de- Often a boiler will be shut down altogetherfor any of a number of reasons. During this period sulfuric acid whichdeposited on the metallic surfaces while the flue gases were cooling mayseverely attack the metallic surfaces in the flue stack.

In the past it has been the practice to wash the ferrous sulfate fromthe metallic surfaces from time to time with aqueous sodium hydroxide orother caustic. Depending upon the particular fuels utilized, thecombustion efficiency, the length of the stack and other operatingfactors it is often necessary to shut down a boiler for a caustic washas often as once a week. Only rarely is it possible to go for a periodof six weeks or more without a caustic wash. This procedure isinefiicient since it takes the boiler out of service, it is costly and,more important, it does not prevent corrosion, it simply removes theproducts of the corrosion.

It is known to use tertiary amines.particularly coal tar bases obtainedby distillation of crude coal tar to inhibit the corrosion of metallicsurfaces which come in contact with combustion products. These coal tarbases are principally impure mixtures containing pyridine and itshomologs such as collidine and ethyl pyridine as the compounds. Theresults which can be obtained by the use of tertiary amines are notcompletely satisfactory since relatively large quantities of coal tarbases must be employed. Moreover, pyridine and its homologs areparticularly characterized by their very noxious odor which occasions anair pollution problem.

It has now been discovered in accordance with this invention thatcorrosion of the type described above can be controlled by condensing orotherwise depositing a small quantity of a selected quaternary ammoniumsalt or salts on surfaces which are reactive with sulfuric acid.Extremely minute quantities of the selected salt may be used. It hasbeen found in fact that as little as from 1X10 lbs. per sq. ft. to l 10lbs. per sq. ft. deposited on the area to be protected will effectivelyinhibit acid corrosion for extended periods of time. This quantity ofsalt is markedly below the quantity normally employed with tertiaryamines. It is such a small quantity relative to the amount of sulfuricacid deposited that it clearly is not a neutralization reaction.Deposition of as little as 40 to 60 parts of salt per million parts ofsulfur containing combustion fuel will extend the period between therequirement for caustic washing of air heaters economizers or otherdevices in the flue stack by as much as ten times or even more.

A number of suitable quaternary salts are available and may be employedin the process of this invention. The

particular salt or salt mixture may be selected from those quaternaryammonium salts which are stable and substantially non-volatile at thedew point of the combustion products. Thus salts which retain theirchemical identity and are solid or liquid at temperatures of about 350F. to about 400 F. may be employed in the practice of this invention.Quaternary ammonium hydroxides and quaternary ammonium halides are as aclass generally not preferred for use in this invention, since many ofthem do not have the desired characteristics with respect to stabilityand non-volatility. Particularly preferred quaternary ammonium saltsinclude tri-lower alkyl-monohigher alkyl sulfonates especiallypara-toluene sulfonates in which the total number of carbon atoms in thehigher alkyl groups is about sixteen to twenty. Typical quaternary saltswhich may be used in the invention include cetyl trimethyl ammonium paratoluene sulfonate and stearyl trimethyl ammonium para toluene sulfonate.The brand of cetyl trimethyl ammonium para toluene sulfonate availableunder the trade name CETATS from Fine Organics, Inc., of Lodi, NewJersey, has been found to be especially suitable.

The selected corrosion inhibitor is conveniently introduced in aqueoussuspension or solution utilizing hoses, spray, or other equivalent meansof distribution. The mixture may be deposited on the surfaces to beprotected during a shut down period. Alternatively it may be depositedduring the start up period of the boiler when the temperature of theflow gases is increasing or during a banking operation when theirtemperature is decreasing. In the latter event it is best to introducethe corrosion inhibitor during the period when the flue gases are at ornear the dew point. The inhibitor may also be introduced at a point inthe flue stack where the flue temperature of the flue gases is above thedecomposition temperature of the quaternary salts or above a point whereconsiderable quantities of the inhibitor would be lost by evaporation.However, if this procedure is used the velocity of the flue gases mustbe such that the inhibitor is rapidly carried to the surface to beprotected before the quaternary salt reaches its decompositiontemperature or a temperature at which subtsantial quantities are lostdue to volatilization.

Any convenient injection means may be employed, for example, a spraynozzle located at or near the area to be protected. The spray isdiffused into the flue gases and thereby effectively dispersed over thesurface of the economizer, air heater or comparable device. Suitableaqueous mixtures for use in the invention may contain from about 0.003%to about 1.5 by Weight of quaternary salts.

The following non-limiting examples illustrate the process as appliedfor the protection of economizers or air heaters, although of course theinvention is not limited to the protection of such devices.

Example 1 This example was carried out on commercial boilers whichnormally operate at a steaming rate of about 200,000 lbs. per hour andare equipped with plate-type air heaters. When the boilers were comingoff the line in the evening and the steaming rate was below 90,000 lbs.per hour at 0.003% by weight solution containing 85 grams of cetyltrimethyl para-toluene ammonium sulfonate in 750 gallons of water wasinjected into the flue gas passage ahead of the air heater during aperiod of to minutes. The area of the air heater surface to be protectedis about 27,000 sq. ft. Accordingly, the addition of the 0.003% solutionis the equivalent of 0000007 lb. of inhibitor per sq. ft. of heatingarea. Based on the estimated oil burned during the period ofintroduction of the inhibitor 0.00035 lb. per gallon of oil or 0.000044lb. per pound of oil (44 parts per million). Flue gas temperatures atthe inlet air heater range from 500 to 600 F. and at the outlet airheater 350 to 450 F. Prior to this treatment it was normally necessaryto remove ferrous sulfate by treatment with alkali about every threeweeks. Treatment in accordance with this example has extended the timeperiod between alkali washings for as long as 8 weeks or more.

Example 2 The commercial type steam boiler which normally oper' ates ata steaming rate of about 350,000 lbs. per hour and is equipped withsteel tube economizers was used in this example. During the bankingperiod when the boiler was down to about 100,000 lbs. of steam per houra 0.1% solution of cetyl trimethyl ammonium para toluene sulfatecontaining 4 lbs. of inhibitor dissolved in 480 gallons of water waspumped into the stack at a rate of 60 gallons per hour for 1 hour. Hence0.5 lb. of the inhibitor was employed. To insure coverage of the entireeconomizer two points of addition were used. The economizer surface iscalculated to be about 16.000 to 17,000 sq. ft. The lower section of theeconomizer, about 11,000 sq. ft. was treated with 0.00002 lb. ofinhibitor per sq. ft. The upper section where corrosion is most severewas treated with 0.00005 lb. per sq. ft. Based on estimated oil burnedduring the introduction period the total amount of inhibitor is 0.000043lb. per gallon of oil or 0.000054 lb. per lb. of oil (54 parts permillion). Flue gas temperatures in and out of the economizer areapproximately the same as in Example 1. This boiler was operated for sixweeks without any indication of a requirement for a soda ash treatment.

Similar results are obtained with stearyl trimethyl paratoluenesulfonate and with mixtures of cetyl and stearyl trimethyl para toluenesulfonate.

What is claimed is:

1. A method of inhibiting corrosion of metallic surfaces caused bydeposition of condensed aqueous sulfuric acid solutions on said surfacesfrom the combustion products arising from combustion ofsulfur-containing fuels, which method comprises depositing on saidsurfaces a quaternary ammonium salt which is substantially non-volatileand stable over the temperature range of from about 350 to 400 F. atwhich the sulfuric acid solution condenses from said combustionproducts, said salt being deposited in a small quantity sufficient toinhibit said corrosion.

2. A method according to claim 1 in which the quaternary ammonium saltis deposited by injecting an aqueous mixture containing from 0.003 toabout 1.5% by weight of the inhibitor into the flue gases during theperiod while their temperature is increasing, but before the temperatureexceeds the dew point.

3. A method according to claim 1 in which the quater nary ammonium saltis deposited by injecting an aqueous mixture containing from about0.003% to about 1.5% of the salt into the flue gases during a periodwhile the fuel combustion is being decreased, but before the flue gaseshave cooled to the dew point.

4. The method according to claim 1 in which the quaternary ammonium saltis deposited during a period when fuel combustion is discontinued.

5. A method of inhibiting corrosion of metallic surfaces caused bydeposition of condensed aqueous sulfuric acid solutions on said surfacesfrom the combustion products arising from combustion ofsulfur-containing fuels, which method comprises depositing on saidsurfaces a quaternary ammonium salt which is selected from the groupconsisting of cetyl and stearyl trimethyl ammonium para-toluenesulfonates and mixtures thereof, said salt being deposited in a smallquantity sufiicient to inhibit said corrosion.

6. A method in accordance with claim 5 in which the quantity of saidsalt which is deposited is from 1 10 to about 1X 10- lbs. per sq. ft. ofsurface.

7. A method in accordance with claim 5 in which the quaternary ammoniumsalt is deposited by injecting an aqueous mixture containing from 0.003%to about 1.5 by weight of the inhibitor into the flue gases during theperiod while their temperature is increasing, but before ReferencesCited the temperature exceeds the dew point.

8. A method in accordance With claim 5 in which the UNITED STATESPATENTS quaternary ammonium salt is deposited by injecting an 3,306,2352/1967 Lewls 6t XR aqueous mixture containing from about 0.003% to about5 1.5% of the salt into the flue gases during a period while FOREIGNPATENTS the fuel combustion is being decreased, but before the 734,1907/1955 Great Britain.

flue gases have cooled to the dew point.

9. A method in accordance with claim 5 in which the JAMES SEIDLECK,Primary Examine!- quaternary ammonium salt is deposited during a period10 H AYES, Assistant Examiner when fuel combustion is discontinued.

